The present invention relates generally to particle sizing instruments, and more specifically to a device for determining the particle size distribution in a liquid or a gas using Fraunhofer diffraction patterns.
Many processes would benefit from on-line monitoring of liquid and gaseous suspensions. For example, the ability to characterize the size distribution of dispersed particles and droplets is of crucial importance in a number of practical systems. Some important applications include liquid fuel droplets sprayed into air in combustion systems such as boilers and gas turbine combustors; solid particles dispersed in liquids as in coal-oil slurries; solid particles dispersed in combustion exhausts with respect to the health aspects of particulate pollutant emissions; and others. In many of these applications optical (as opposed to batch) sampling techniques for particle sizing are advantageous and sometimes necessary. (The term particle will refer herein to both solid particulate matter and liquid droplets of diameters approximately 0.01 .mu.m to 1 mm.)
A problem which is often encountered in measuring techniques is to determine the size distribution of physical entities, such as particles in a liquid or gas. This task is alleviated, to some extent, by the systems described in the following U.S. Patents, the disclosures of which are specifically incorporated herein by reference:
U.S. Pat. No. 3,469,921 issued to Taylor; PA1 U.S. Pat. No. 3,636,367 issued to Girard; PA1 U.S. Pat. No. 4,037,964 issued to Wertheimer et al; PA1 U.S. Pat. No. 4,338,030 issued to Loos; PA1 U.S. Pat. No. 4,251,733 issued to Hirleman; PA1 U.S. Pat. No. 4,188,121 issued to Hirleman; PA1 U.S. Pat. No. 3,835,315 issued to Gravitt; PA1 U.S. Pat. No. 3,689,772 issued to George et al; PA1 U.S. Pat. No. 3,988,612 issued to Palmer; PA1 U.S. Pat. No. 4,360,799 issued to Leighty; and PA1 U.S. Pat. No. 4,740,677 issued to Carreras et al.
Advanced optical systems for determining the particle parameter of size often use laser illumination of single particles and analysis of the scattered light characteristics to obtain information on the size and other physical parameters of a given particle. The sizes of many particles are measured and summed to determine an overall particulate size distribution. The use of lasers is advantageous due to the greater light intensity available as compared to conventional light sources, thereby allowing measurement of smaller particles and enhancing the ability for in-situ or non-interfering measurements. Arrangements using white light scattered in only one solid angle require an extremely well defined and compact sampling volume through which a representative sample of the particulate flow must be passed.
In the system disclosed by Gravitt, laser or other light is focused to intensely illuminate a small region in space. This region, called the sensitive volume or particle sampling zone, is located in the field of light collecting apparatus which discriminates between the light scattered at two small angles and the light traveling in the light beam propagation direction. Detector means are used simultaneously to detect and record signals representing the intensities of the scattered light detected at the different angles. A measure of one of the parameters, i.e. the particle size, of a particle passing through the sampling zone is determined by measuring the ratio of the signals representing the intensities of the scattered light detected at two angles. This measurement is, however, non-unique or ambiguous since particles of different sizes may pass through the sampling zone and since many particle sizes can generate the same ratio signal.
One problem with a laser system is the Gaussian intensity distribution in the beam, since single angle systems can not differentiate between a small particle passing through the high-intensity center of the beam and a larger particle passing through an off-center point of lower intensity. This problem can be eliminated by utilizing the ratio of light intensities scattered in two directions thereby cancelling the incident intensity effect as suggested by Gravitt.
Since the Fraunhofer diffraction pattern possesses circular symmetry, the rings and wedges sample the diffracted energy in polar coordinate form. That is, the rings sample the distance of the diffraction pattern portions from the axis, while the wedges sample the direction at which portions of the pattern are disposed. A suitable wedge-ring Detector, having 32 rings and 32 wedges, is disclosed in U.S. Pat. No. 3,689,772, and is manufactured by Recognition Systems, Inc. of Van Nuys, Calif.
The above-cited Palmer reference discloses a photodetector array system, which is an array which is comprised of a matrix of photodiode detectors, and may for instance be a 32 by 32 element device such as the Reticon model R32X32A.
The first of the above-cited Hirleman patents disclose a technique for measuring particle size and velocity, using two beams of electromagnetic radiation with symmetric radial intensity distributions are directed through space. A particle sampling volume is defined by those portions of the two beams within the field of view of one or more radiation sensitive detectors. The detectors respond to scattered radiation or fluorescence from particles passing through the beams in the sampling volume. The detector output for a single particle indicates two signal pulses corresponding to those times when the particle was in one of the beams. The speed of the particle in the plane perpendicular to the beams is determined from the transit time or width of the signal pulses, and the angle of the particle traverse in that plane determined from the time-of-flight between the signal pulses.
The second of the above-cited Hirleman patents is an improved multiple ratio single particle counter. Intensities of scattered radiation are measured at more than two angles and ratios of these intensities are derived. These ratios are compared with calibration curves to determine an unambiguous measure of the particle parameter.
Loos shows an arrangement for measuring the size distribution of particles suspended in a gas or in a liquid. In this patent a spatial filter is placed in the exit plane of a dispersive element so that its transmittance is a function of position on the filter. Light transmitted by the filter is measured by a photodetector. The photodetector output is measured as different spatial filters are switched in place.
Wertheimer et al discuss a Fraunhofer plane spatial filter in which a mask lies in the Fraunhofer plane of lens. In Taylor size distribution of an aggregation is determined by the amount of light in a ring in the Fourier plane. Girard describes a Fourier transform optical analyzer which uses a mask shifted step by step relative to an optical object support.
While the above-cited references are instructive, the task of measuring particle size distribution in liquids and in gas remains an ongoing need. The present invention is intended to make a useful contribution towards satisfying that need.